This invention relates generally to mixers of low distortion type, which are used for tuners or the like of cable television receivers.
In conventional mixers of this type, when transmitting power in VHF-UHF bands from an unbalanced circuit to a balanced circuit, an unbalanced current occurs at the connecting portion between these circuits and therefore spurious radiation and mismatching result. For this reason, a balun (Balanced to Unbalanced) is used to remove such drawback.
Most conventional baluns have a structure such that wires are wound spirally around a core of high permeability. However, such baluns suffer from considerable conversion loss when the frequency of an input signal is high. Furthemore, it is difficult to set the length of the wires to be wound so that phase difference between in-phase signal and antiphase signal is accurately 180 degrees. For these reasons, baluns are actualized recently by way of conductor patterns formed on a substrate without using wound wires.
However, such known conductor-pattern type baluns suffer from insufficient characteristics in connection with second order intermodulation distortion, while it is difficult to be mass produced with high accuracy as will be described in detail hereinlater.